Rich Benner hires some movers:
As consultants, we often see system databases existing on the C drive on SQL Servers. There are some issues with this setup, and the biggest is: if one of your system databases grows and fills your C drive, you will likely crash the OS. If that happens, we’re in big trouble. Therefore, moving system databases becomes a necessary operation at times.
This is such a common issue because the default locations are set to C for these databases and that’s where they end up on fresh installs 99% of the time. Don’t worry! If you’re in this situation you’re not alone.
Click through for a query that shows which databases are on which drive and how to migrate databases post-install.